**4. Discussion of Research Results**

The results of the XPS analysis allow us to judge the chemical composition and electronic state of atoms on the surface of the formed DLCs. From the experimental data obtained in Tables 4 and 5, it can be seen that DLCs of the two types studied (without sublayer and with sublayer (CrAlSi)N) have a very close ratio of components and approximately 64–66% consist of carbon atoms with different hybridization of valence electrons: *sp*3-hybridization (diamond structure) and *sp*2-hybridization (graphite structure). At the same time, the proportion of diamond hybridization (which is of particular interest to researchers) is almost identical for a single-layer DLC and a double-layer (CrAlSi)N/DLC. It is 47% and 51%, respectively, and the *sp*3/*sp*<sup>2</sup> ratios of these coatings are similar at 3.0 and 3.4, respectively. First, these indicators show good reproducibility of PECVD technology used in work for the deposition of DLCs on a hard alloy. We add that this technology in a previous work was used by the authors for the deposition of DLC films on oxide-carbide ceramics [26,62,63], and the value of the *sp*3/*sp*<sup>2</sup> ratio was close to that which was established when coating a hard alloy. Secondly, taking into account the fact that the volume fractions of the revealed silicon compounds on the surface of the DLC and (CrAlSi)N/DLCs were very close, which, taking into account the analysis error, allows us to conclude that the physicochemical processes occurring under the temperature conditions of PECVD technology do not lead to intensive chemical adsorption, mixing, and the formation of new phases in the formed DLC film. This is also indicated by the absence of compounds based on tungsten (the main component of the hard alloy), chromium, and aluminum (components of the CrAlSi sublayer) on the surface of DLCs, and the presence of silicon compounds in the two coatings are the result of discharge destruction of tetramethylsilane, which is a component of the gas mixture. It is necessary to clarify, speaking about the absence of chemisorption, we mean volumetric interaction processes inside the coating layer, while understanding that there is a chemical interaction at the substrate/coating interface, since it would be impossible to ensure good adhesive bond strength in its absence.

Thus, in reality, the structure (chemical composition and electronic state of atoms) of the formed DLC does not depend on the chemical composition and properties of the substrate on which it is deposited but is determined by the strategy and parameters of the implementation of PECVD technology. At the same time, studies have shown that the physical and mechanical characteristics and strength of the adhesive bond of the formed DLC, on the contrary, strongly depend on the properties of the substrate on which they are deposited. Therefore, DLC films that are very similar in chemical composition and electronic state of atoms can have very different values of physical and mechanical characteristics, particularly the modulus of elasticity (Table 6).

The Martens hardness values measured by nanoindentation were very similar (Table 6) for all samples with DLCs (without sublayer and with sublayer (CrAlSi)N). At a load of 1.0 mN, the samples had hardness 33 and 34 GPa, and with increasing loads up to 4.0 mN and, respectively, with the increase in the contact depth, the hardness of samples decreased to 25 and 26 GPa. Interesting regularities were established concerning the quantitative values of the modulus of elasticity of hard alloy samples with different coatings. The data on the modulus of elasticity (*E*) of samples calculated during nanoindentation and shown in Table 6 essentially characterize the degree of rigidity of the surface layer of the tool material and its ability to deform elastically under external loading. We know that the lower the modulus of elasticity, the less rigid the material is with low deformation resistance, but it is less brittle and more ductile. If at a low load (1.0 mN), the *E* values for all samples were similar, 292 GPa for the DLC and 285 GPa for (CrAlSi)N/DLC, then at a higher load (4.0 mN), significant differences were found, 242 and 201 GPa. Analysis of the obtained data suggests that it is possible to identify areas in which there is a different nature of changes in the properties of the coating. For example, when an indenter is inserted to a depth of 90–100 nm, the properties of DLCs are determined by the surface characteristics. If the indenter is embedded to a depth of 380 nm or more, the properties of DLCs largely depend on the state and properties of the substrate and, of course, on the stresses at the "substrate-coating" interface. Hardness and modulus of elasticity cannot be considered separately, since they are interrelated, and their *H*/*E* ratio carries information about an important characteristic—the index of plasticity. A high value of the index of plasticity provides an increased service life in conditions of cyclic loads typical for milling, and the closeness of the values of the modulus of elasticity of the coating and the substrate helps to reduce technological stresses on the interface and increase the adhesive strength. Authoritative studies show that the ratio *H*/*E* = 0.15 characterizes the "ideal elasticity" of the coating [64–66]. In the present study, this indicator at a load of 1.0 mN was similar for DLC and (CrAlSi)N/DLCs, 0.113 and 0.119, respectively. In tests with a fourfold increase in load, the influence of the properties of the substrate with a preformed sublayer (CrAlSi)N was immediately indicated, the index of plasticity was almost 0.13, while for a single-layer DLC, this indicator was at the level of 0.1.

It seems logical to explain that the higher plasticity of the surface layer (coating) of the samples and the less stressed state at the "substrate-coating" interface had a decisive effect on the subsequently established higher adhesive bond strength of the DLC deposited on a carbide substrate with a pre-formed sublayer (CrAlSi)N. Of course, the hardness of the formed coating is a primary characteristic, but it is the elastic characteristics of the substrate and coating that have the most significant influence on the level of occurring stresses [67–70]. As a confirmation of this, we can consider the empirical dependence of the stresses in the coating, which appear due to the loading of the substrate, when tangential stresses occur in the plane of the adhesive contact, which can cause adhesive destruction of the coating. The maximum tangential stresses (τ*max*) in the plane of the adhesive contact are equal to [28]:

$$
\tau\_{\text{max}} = P \frac{L}{Ek} \tag{2}
$$

where *P* is the load applied to the substrate (Pa); *E* is the modulus of elasticity of the substrate (GPa); *L* (Pa/m) and *k* (m<sup>−</sup>1) are coefficients that depend on the modulus of elasticity of the coating.

It can be seen that by reducing the modulus of elasticity of the coating, it is possible to reduce the values of the maximum tangential stresses, the effect of which leads to the delamination of the coating, making it possible to increase its resistance to acting loads.

It is no coincidence that the experimental data shown in Figure 7 demonstrated that peeling of local areas of the coating from the substrate and especially the separation of the coating from a sample with a single-layer DLC, which has a higher modulus of elasticity, occurs at a significantly (almost two times) lower average force, at 12 N, while for the sample with (CrAlSi)N/DLC, which has a lower modulus of elasticity, at a load of 23 N.

The results of a comparative qualitative assessment of the adhesive bond strength of various coatings with hard alloy substrates once again confirm the above (Figure 8). Since local elastic-plastic deformation occurs around the indenter print during indentation, which causes swelling and destruction of the coating, it is the plasticity of the coating, which directly depends on the modulus of elasticity that is of paramount importance. A less plastic single-layer DLC at a uniform distance from the hole formed by the indenter has local areas of peeling of the coating and micro-chips, while a more plastic (CrAlSi)N/DLC after the load is removed, restores the surface layer, and there are no irreversible changes in it.

Since the resistance of the coating to abrasive wear is an important indicator that largely determines the performance of a coated product during operation, it is also necessary to discuss the results presented in Figure 9. The obtained images make it possible to approximate the surface area worn by external abrasive action, which can approximate the wear intensity. Unexpected results were not obtained here; the wear rate of hard alloy (CrAlSi)N/DLC-coated samples is 2.4 times lower than the uncoated samples and 1.5 times lower than the wear of single-layer DLC-coated samples.

Given that the hardness, which is the leading property that provides resistance to abrasion, two different coatings had very similar values (Table 6), such results can be explained as follows. First, it is the higher adhesive bond strength of the (CrAlSi)N/DLC with the substrate already discussed above. Secondly, it is necessary to consider the fact that after the external DLC layer is worn off; the nitride sublayer continues to function for some time and performs protective functions. Furthermore, the total thickness of (CrAlSi)N/DLCs is 4 μm, and the thickness of a single-layer DLC is 2.6 μm, which also affects the intensity of abrasive wear.

All of the above experimental results are very indicative and explain a lot, but they cannot consider the whole complex of actual operating loads. Therefore, it is possible to conclude the effectiveness of a particular coating only after discussing the results of operational tests of end mills with two types of DLCs when machining aluminum alloy and structural steel. In the operational tests carried out, the coatings showed significant differences in efficiency. The fact that when processing the aluminum alloy AlCuMg2, the application of a single-layer DLC significantly increased the durability of the end mill, and the application of a sublayer (CrAlSi)N did not contribute to an additional increase in the durability of the DLC-coated tool (Figure 10) can be explained as follows. The main task of the coating when processing aluminum-based alloys is to reduce the friction interaction on the contact pads of the cutter since the tool wear mechanism is associated with active cyclic adhesion of aluminum particles. The separation of particles from the contact surface of the tool and the workpiece is accompanied by the removal of micro-volumes of tool material from the working surfaces of the cutter, which are carried away from the cutting area by descending chips. However, a significant proportion sticks to the opposite surface of the part, reducing the quality of the treated surface. Therefore, due to the external DLC, which has excellent anti-friction characteristics, there is a decrease in friction interaction on the contact pads of the mill, and the development of the wear chamfer on the flank surface is significantly slowed down (Figures 10 and 11). The power loads when milling aluminum alloy are such that the coating on the cutting tool works in relatively favorable conditions and, according to experimental data (Figure 10), there is no need to form a sublayer under the external DLC (CrAlSi)N, thereby affecting the resistance of the coating to elastic and plastic deformations.

Analysis of the experimental data (Figure 13) obtained during the processing of 41Cr4 structural steel allows us to draw conclusions that differ significantly from those made above. This is not surprising, given that the maximum values of cutting forces that occur when processing structural steels can be four times higher than the corresponding values of force parameters when processing aluminum alloys. Judging by the experimentally obtained dependences of tool wear on operating time (Figures 14 and 15), the external DLC quickly ceases to perform protective functions, and the conditions of contact interaction on the front and flank surfaces of the tool are similar to those that occur when using end mills without coatings. The role of the (CrAlSi)N sublayer under the influence of heat and power loads when milling 41Cr4 steel is manifested in the fact that more favorable conditions are created for the functioning of the external DLC, its adhesive bond strength with the tool material increases, the stress level in the coating decreases and it is capable of resisting the current heat and power loads for a longer time. Even after the abrasion of the outer DLC layer, the nitride sublayer can act as a wear-resistant coating, slowing down the wear rate. A consequence of the described processes is an increase in the resistance of the (CrAlSi)N/DLC to the applied loads and providing a twofold increase in the wear resistance of end mills compared to an uncoated tool when milling 41Cr4 steel (Figure 13) and 1.8 times compared to milling cutters with a single layer DLC.

The experimentally obtained dependences of the roughness of the machined surfaces of workpieces made of aluminum alloy AlCuMg2 and structural steel 41Cr4 on the operating time of end mills with various DLCs, presented in Figures 12 and 15, are in good agreement with the characteristic curves of the development of mill wear in time discussed above (Figures 10 and 13) and obey the classical principles of the theory of metal cutting. The wear of the cutting tool leads to an increase in the roughness of the machined surface because there is an increase in the actual contact area of the cutting tool and the workpiece being processed, and the intensity of their frictional and adhesive interaction increases. The use of DLCs hinders the development of these processes (Figure 12), thereby significantly reducing the average height of microroughness of the surface layer of an aluminum alloy part, which, even at the moment of tool failure (i.e., with a heavily worn tool), did not exceed 2.5 μm. In contrast to the milling of aluminum alloys during the forming of structural steel 41Cr4, the contribution of DLCs is significantly lower in reducing the intensity of friction and adhesive interaction between the tool and the workpiece. Therefore the effect on the roughness of the treated surface is not so pronounced. Nevertheless, the coating (CrAlSi)N/DLC slightly reduces the average height of the micro-roughness of the surface layer of the structural steel part (Figure 15), which is 2.5 μm in the first minutes of tool operation, and 8.75 μm at the time of tool failure.
